Characterization of micro- and nanophase separation of dentin bonding agents by stereoscopy and atomic force microscopy

Influence of Surface Pretreatment on the Bond Strength of a Resin Luting Cement to Saliva-contaminated Enamel and Dentin

OBJECTIVES

This study aimed to evaluate the influence of surface pretreatment on the shear bond strength (SBS) of a resin luting cement to enamel and dentin with saliva contamination. The surface free energies (SFE) of the adherent surfaces were also determined.

METHODS AND MATERIALS

Bovine enamel and dentin were used in this study. For the saliva-contamination, human saliva was applied to the adherent surface for 60 seconds and then air-dried, and the specimens without saliva contamination served as controls. One group of contaminated surfaces was untreated (SC), and the others were pretreated with Katana Cleaner (KC), Multi Etchant (ME), or Ultra-Etch (UE). Fifteen specimens were prepared to measure the SBS for each test group.The mixed resin luting cement paste was applied to the alumina-blasted surface of a stainless-steel rod and placed on the prepared tooth surface. The luting cement was light irradiated for 40 seconds. The bonded specimens were stored for 24 hours at 37°C and half of the bonded specimens underwent 10,000 thermal cycles. The SBS and SFE of the specimens after different pre-treatments were measured.

RESULTS

The two-way ANOVA revealed that the factors of pretreatment agent and storage condition had a significant effect on the SBS to enamel and dentin. The SFE values of the SC group were significantly lower than those of the other groups in both enamel and dentin. The SFE of pretreated surface was material dependent.

CONCLUSIONS

A pretreatment agent containing functional monomers was shown to be effective in removing saliva contaminants and in creating an effective bonding surface for the resin luting cement.

Compatibility versus reaction diffusion: Factors that determine the heterogeneity of polymerized adhesive networks

OBJECTIVES

Heterogeneity and phase separation during network polymerization is a major issue contributing to the failure of dental adhesives. This study investigates how the ratio of hydrophobic crosslinkers to hydrophilic comonomer (C/H ratio), as well as cosolvent fraction (ethanol/water) influences the degree of heterogeneity and proclivity for phase separation in a series of model adhesive formulations.

METHODS

Twelve formulations were investigated, with 4 different C/H ratios (7:1, 2.2:1, 1:1, 0.5:1) and 3 different overall cosolvent fractions (0, 10 and 20 wt%). The heterogeneity and phase behavior were characterized using Fourier Transform Infrared Spectroscopy (FT-IR), dynamic mechanical analysis (DMA), small-angle x-ray scattering (SAXS) and atomic force microscopy (AFM).

RESULTS

In resins without cosolvent, all characterizations confirm reduced heterogeneity as C/H ratio decreases. However, when 10 or 20 wt% of cosolvent is included in the adhesive formulation, a higher degree of heterogeneity and even distinct phase separation with domains ranging from a few hundreds of nanometers to a few micrometers in size form. This is particularly noticeable at lower C/H ratios, which is surprising as HEMA is commonly considered a compatibilizer between hydrophobic crosslinkers and aqueous (co)solvents.

SIGNIFICANCE

Our experiments demonstrate that formulations with lower C/H ratio and thus a lower viscosity experience later onsets of diffusion limitations during polymerization, which favors thermodynamically driven phase separation. Therefore, to determine or predict the resulting phase structure of adhesive materials, it is necessary to consider the kinetics and diffusion constraints during the formation of the polymer network and not just the compatibility of resin constituents.

Autonomous-Strengthening Adhesive Provides Hydrolysis-Resistance and Enhanced Mechanical Properties in Wet Conditions

The low-viscosity adhesive that is used to bond composite restorative materials to the tooth is readily damaged by acids, enzymes, and oral fluids. Bacteria infiltrate the resulting gaps at the composite/tooth interface, demineralize the tooth, and further erode the adhesive. This paper presents the preparation and characterization of a low-crosslink-density hydrophilic adhesive that capitalizes on sol-gel reactions and free-radical polymerization to resist hydrolysis and provide enhanced mechanical properties in wet environments. Polymerization behavior, water sorption, and leachates were investigated. Dynamic mechanical analyses (DMA) were conducted using water-saturated adhesives to mimic load transfer in wet conditions. Data from all tests were analyzed using appropriate statistical tests (α = 0.05). The degree of conversion was comparable for experimental and control adhesives at 88.3 and 84.3%, respectively. HEMA leachate was significantly lower for the experimental (2.9 wt%) compared to control (7.2 wt%). After 3 days of aqueous aging, the storage and rubbery moduli and the glass transition temperature of the experimental adhesive (57.5MPa, 12.8MPa, and 38.7 °C, respectively) were significantly higher than control (7.4MPa, 4.3 MPa, and 25.9 °C, respectively). The results indicated that the autonomic sol-gel reaction continues in the wet environment, leading to intrinsic reinforcement of the polymer network, improved hydrolytic stability, and enhanced mechanical properties.

Hydrophilic dyes as photosensitizers for photopolymerization of dental adhesives

OBJECTIVES

This study explored hydrophilic dyes as photosensitizers for application in dental adhesives. The goal was to identify dyes that enhance the degree of conversion (DC) of the hydrophilic-rich phase without impairing polymerization of the hydrophobic-rich phase.

METHODS

Properties that were investigated included the molar extinction coefficient at 480 nm, relative normalized photon absorption efficiency (PAE), rate of polymerization and degree of conversion (DC). The following hydrophilic dyes: Bromophenol blue sodium salt, Rosebengal sodium salt, Erythrosin B, New Fuchsin and Victoria blue B were identified as suitable photosensitizers.

RESULTS

In this study it was observed that dyes such as Bromophenol blue sodium salt, New Fuchsin, Victoria blue B and Rosebengal sodium salt were suitable candidates for dental adhesive photopolymerization, leading to substantial degree of conversion to both the hydrophilic-rich phase and the hydrophobic-rich phase.

CONCLUSIONS

In addition to the ability of the photosensitizer to absorb light in the visible range and transition to an excited state as a result of the absorbed energy, other factors such as the efficiency of the photosensitizer/light curing unit (LCU) combination, stability/efficiency of the excited state of the photosensitizer and/or initiating reactive species play an important role in the photopolymerization of the dental adhesive.

No-waiting dentine self-etch concept-Merit or hype

OBJECTIVE

A recently-launched universal adhesive, G-Premio Bond, provides clinicians with the alternative to use the self-etch technique for bonding to dentine without waiting for the adhesive to interact with the bonding substrate (no-waiting self-etch; Japanese brochure), or after leaving the adhesive undisturbed for 10s (10-s self-etch; international brochure). The present study was performed to examine in vitro performance of this new universal adhesive bonded to human coronal dentine using the two alternative self-etch modes.

METHODS

One hundred and ten specimens were bonded using two self-etch application modes and examined with or without thermomechanical cycling (10,000 thermal cycles and 240,000 mechanical cycles) to simulate one year of intraoral functioning. The bonded specimens were sectioned for microtensile bond testing, ultrastructural and nanoleakage examination using transmission electron microscopy. Changes in the composition of mineralised dentine after adhesive application were examined using Fourier transform infrared spectroscopy.

RESULTS

Both reduced application time and thermomechanical cycling resulted in significantly lower bond strengths, thinner hybrid layers, and significantly more extensive nanoleakage after thermomechanical cycling. Using the conventional 10-s application time improved bonding performance when compared with the no-waiting self-etch technique. Nevertheless, nanoleakage was generally extensive under all testing parameters employed for examining the adhesive.

CONCLUSION

Although sufficient bond strength to dentine may be achieved using the present universal adhesive in the no-waiting self-etch mode that does not require clinicians to wait prior to polymerisation of the adhesive, this self-etch concept requires further technological refinement before it can be recommended as a clinical technique.

CLINICAL SIGNIFICANCE

Although the surge for cutting application time to increase user friendliness remains the most frequently sought conduit for advancement of dentine bonding technology, the use of the present universal adhesive in the no-waiting self-etch mode may not represent the best use of the adhesive.

Effect of Partition of Photo-initiator Components and Addition of Iodonium Salt on the Photopolymerization of Phase-Separated Dental Adhesive

The polymerization kinetics of physically separated hydrophobic- and hydrophilic-rich phases of a model dental adhesive have been investigated. The two phases were prepared from neat resin containing 2-hydroxyethyl methacrylate (HEMA) and bisphenol A glycerolate dimethacrylate (BisGMA) in the ratio of 45:55 (wt/wt). Neat resins containing various combinations of popular photo-initiating compounds, e.g., camphoquinone (CQ), ethyl 4-(dimethylamino)benzoate (EDMAB), 2-(dimethylamino)ethyl methacrylate (DMAEMA) and diphenyliodonium hexafluorophosphate (DPIHP) were prepared. To obtain the two phases 33 wt% of deuterium oxide (D₂O) was added to the neat resins. This amount of D₂O exceeded the miscibility limit for the resins. The concentration of each component of the photo-initiating system in the two phases was quantified by HPLC. When combined with CQ, DMAEMA is less efficient as a co-initiator compared to EDMAB. The addition of DPIHP as the third component into either CQ/EDMAB or CQ/DMAEMA photo-initiating systems leads to comparable performance in both the hydrophobic- and hydrophilic-rich phases. The addition of the iodonium salt significantly improved the photopolymerization of the hydrophilic-rich phase; the hydrophilic-rich phase exhibited extremely poor polymerization when the iodonium salt was not included in the formulation. The partition concentration of EDMAB in the hydrophilic-rich phase was significantly lower than that of DMAEMA or DPIHP. This study indicates the need for a combination of hydrophobic/hydrophilic photosensitizer and addition of iodonium salt to improve polymerization within the hydrophilic-rich phase of the dental adhesive.

Impact of light intensity on the polymerization kinetics and network structure of model hydrophobic and hydrophilic methacrylate based dental adhesive resin

The impact of light intensity on the degree of conversion (DC), rate of polymerization and network structure was investigated for hydrophobic and hydrophilic dental adhesive resins. Two and three component photoinitiating (PI) systems were used in this study. Low light intensities had a negative impact on the polymerization efficiency for the hydrophilic resin with 2 component PI system. Incorporation of iodonium salt in the hydrophilic resin significantly improved the polymerization efficiency of the HEMA/BisGMA system and led to a substantial DC, even at low light intensities. The results suggested that shorter polymer chains were formed in the presence of iodonium salt. It appears that there is little or no impact of light intensity on the polymer structure of the 2 component PI system. Light intensity has subtle impact on the polymer structure of the 3 component PI system. In the case of the hydrophobic resin, the polymer is so highly cross-linked that the presence of shorter chains for the 3 component PI system does not cause a decrease in the glass transition temperature (T_g ) when compared to the 2 component PI system. For the hydrophilic resin, the presence of shorter polymer chains in the 3 component PI system reduces the T_g when compared with the corresponding 2 component PI system. © 2015 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 104B: 1666-1678, 2016.

Polymerization behavior of hydrophilic-rich phase of dentin adhesive

The 2-fold objectives of this study were 1) to understand whether model hydrophobic- and hydrophilic-rich phase mimics of dentin adhesive polymerize similarly and 2) to determine which factor, the dimethacrylate component, bisphenol A glycerolate dimethacrylate (BisGMA) or photoinitiator concentration, has greater influence on the polymerization of the hydrophilic-rich phase mimic. Current dentin adhesives are sensitive to moisture, as evidenced by nanoleakage in the hybrid layer and phase separation into hydrophobic- and hydrophilic-rich phases. Phase separation leads to limited availability of the cross-linkable dimethacrylate monomer and hydrophobic photoinitiators within the hydrophilic-rich phase. Model hydrophobic-rich phase was prepared as a single-phase solution by adding maximum wt% deuterium oxide (D2O) to HEMA/BisGMA neat resins containing 45 wt% 2-hydroxyethyl methacrylate (HEMA). Mimics of the hydrophilic-rich phase were prepared similarly but using HEMA/BisGMA neat resins containing 95, 99, 99.5, and 100 wt% HEMA. The hydrophilic-rich mimics were prepared with standard or reduced photoinitiator content. The photoinitiator systems were camphorquinone (CQ)/ethyl 4-(dimethylamino)benzoate (EDMAB) with or without [3-(3, 4-dimethyl-9-oxo-9H-thioxanthen-2-yloxy)-2-hydroxypropyl]trimethylammonium chloride (QTX). The polymerization kinetics was monitored using a Fourier transform infrared spectrophotometer with a time-resolved collection mode. The hydrophobic-rich phase exhibited a significantly higher polymerization rate compared with the hydrophilic-rich phase. Postpolymerization resulting in the secondary rate maxima was observed for the hydrophilic-rich mimic. The hydrophilic-rich mimics with standard photoinitiator concentration but varying cross-linker (BisGMA) content showed postpolymerization and a substantial degree of conversion. In contrast, the corresponding formulations with reduced photoinitiator concentrations exhibited lower polymerization and inhibition/delay of postpolymerization within 2 h. Under conditions relevant to the wet, oral environment, photoinitiator content plays an important role in the polymerization of the hydrophilic-rich phase mimic. Since the hydrophilic-rich phase is primarily water and monomethacrylate monomer (e.g., HEMA as determined previously), substantial polymerization is important to limit the potential toxic response from HEMA leaching into the surrounding tissues.

Polymerization- and solvent-induced phase separation in hydrophilic-rich dentin adhesive mimic

Current dental resin undergoes phase separation into hydrophobic-rich and hydrophilic-rich phases during infiltration of the over-wet demineralized collagen matrix. Such phase separation undermines the integrity and durability of the bond at the composite/tooth interface. This study marks the first time that the polymerization kinetics of model hydrophilic-rich phase of dental adhesive has been determined. Samples were prepared by adding varying water content to neat resins made from 95 and 99 wt.% hydroxyethylmethacrylate and 5 and 1 wt.% (2,2-bis[4-(2-hydroxy-3-methacryloxypropoxy)phenyl1]-propane prior to light curing. Viscosity of the formulations decreased with increased water content. The photopolymerization kinetics study was carried out with a time-resolved Fourier transform infrared spectrometer. All of the samples exhibited two-stage polymerization behavior which has not been reported previously for dental resin formulation. The lowest secondary rate maxima were observed for water contents of 10-30 wt.%. Differential scanning calorimetry (DSC) showed two glass transition temperatures for the hydrophilic-rich phase of dental adhesive. The DSC results indicate that the heterogeneity within the final polymer structure decreased with increasing water content. The results suggest a reaction mechanism involving both polymerization-induced phase separation and solvent-induced phase separation for the model hydrophilic-rich phase of dental resin.

Differential resin-dentin bonds created after caries removal with polymer burs

The objective of this article was to investigate the effect of carbide and polymer burs caries removal methods on the bond strength of different adhesives to dentin. Resin restorations were performed in sound and caries-affected dentin, after using polymer or carbide burs and bonding with four different adhesive (Single bond, SB; Clearfil SE bond, SEB; FL-Bond II, FLB; and Fuji II-LC, FUJI). Microtensile bond strength (MTBS) was measured. Data were analyzed with ANOVA and Student-Newman-Keuls tests. Debonded surfaces were observed by scanning electron microscopy. Bonded interfaces were examined using light microscopy (Masson's trichrome staining). In sound dentin, MTBS was similar for SEB and SB, and higher than that of FLB and FUJI. Bond strength to carbide bur prepared dentin was similar for SB, SEB, and FLB; FUJI presented the lowest. SB applied on polymer bur excavated dentin presented similar values to those of the carbide bur group; MTBS attained by SEB, FLB, and FUJI decreased when bonding to dentin treated with polymer burs; FUJI yielded pretesting failures in all specimens. Polymer burs created a thick smear layer that was not infiltrated by tested self-etching agents. The bonding effectiveness of self-etching and glass-ionomer-like adhesives to dentin decreased when polymer burs were used.